Fuel Line Size - 3/8" ID or 1/2" ID

[22LD]

I was reading the article "The Silence That Stuns" in the June 2016 issue of Sport Aviation (pages 24-26.) The author states for engines 390 cubic inches and above you should run 1/2" I.D. fuel line. I'm pretty sure I read somewhere in Bearhawk documentation that 3/8" was adequate. I have already run my lines with 3/8". The other issue is both my fuel selector valve and boost pump have 3/8" ports. Thoughts? What others done with an O-540 or IO-540? Thanks!

[whee]

Lots of BHs fling with 3/8 fuel lines as Bob recommends. Should be obvious that if Bob's fuel system is followed then one will have adequate fuel flow, but of course one should also preform fuel flow test prior to the first flight.

Here is my thinking for my application:
Fuel tanks have 3/8 fittings, fuel valve has 3/8 fittings, gascolator has 3/8 fittings, fuel pump has 3/8 fittings. There are so many 3/8 fittings in the system that 1/2 fuel line will provide little benefit.

Additionally, looking at pressure loss tables I found that for 3/8" and 1/2" smooth bore "hose" 10ft long flowing 0.5gpm (30gph, representing WOT) has zero pressure loss. This is for a straight section and no fittings. Being that we will have fittings and the section won't be straight there will be some loss and the loss will be less in 1/2" but not that must less because our flow rates aren't that high.

All that being said, I am going to attempt installing 1/2" fuel lines from the tanks to the fuel pump. I'll be installing a constant flow fuel injection system and running mogas so I'm concerned about vapor forming in the fuel line on the suction side of the pump. Rather than installing low pressure fuel pumps in the wing roots to solve this concern I'm going to use larger fuel lines and monitor the fuel pressure closely.

Anyways...that's my thoughts.

[S Lathrop]

I don't think that you would gain much, if anything, by switching to 1/2" lines without changing the size of the fittings into the tanks.

If you changed the lines and fittings to 1/2" from the tanks to the selector valve, you might be able to measure an increase in flow rate through the valve because of the larger column of fuel at the valve. But I doubt that it would be very significant in the overall picture.

I wonder if using a 1/2" line and fittings plus a check valve from the aux tank and tee into the feed lines from the main tanks would not be a way to assure that the fuel from the aux tanks is consumed and not have to use a transfer pumps?

[Zzz]

I've used 3/8" lines but I've intended using electric fuel pumps since the outset. Although, I don't see any disadvantage to using 1/2" other than some negligible weight.

I'm wishing I'd used poly lines now though, like CubCrafters does on their LSAs. There's so much time involved in precisely bending that 5052, and the 3003. Especially if you stake your reputation on its appearance

[nichzimmerman]

I asked Bob about this on the phone once and he told me that the 3/8" lines would provide perfectly adequate flow for this engine, though it might not be the 150% the FAA prescribes. He said there was nothing wrong with using 1/2" lines except that they are harder to bend without kinking.

I'm going to give the 1/2" lines a try. Somewhere in dream land I would love to have good enough flow to put in a gravity fed fuel transducer without requiring a fuel pump. I'm also using the ACS fuel selector with 3/8" NPT ports for 1/2" lines. Remember the old axiom, "The only time you have too much gas is when you are on fire."

[Mark Moyle]

I'll be using 1/2" 5052 line...will spend the money on a good tubing bender. One thought that comes to mind is the 180 hp 0-360 STC for the PA 18, 20 and 22. All three require fuel tank caps with a ram air tube to meet the fuel flow requirements. All three use 3/8" fuel line. Perhaps the reason for the ram air fuel tank caps is that most PA 20's and 22's don't have a both setting on the fuel selector valve....I don't know what fuel valve the super cub has. The other thing to think about is the wall thickness of the tubing.. I mistakenly ordered 3/8" .049 wall tubing. Bends nicely with a good bender...but the STC requires .035" wall tubing.


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[22LD]

Thank you, everyone, for the feedback. My feeling exactly about all the 3/8" fittings/devices in the line. What would be the point of 1/2" line.

[jaredyates]

I think you've already realized it, but let the magazine article be a discussion starter and not a source. Having written a couple of magazine articles and having read plenty of others, the quality of the content is only as good as the writers and editors. I'm far from an expert, and I often read something that makes me raise an eyebrow. I haven't read the article yet, but it sounds more like a rule of thumb than guidance that has been published by a regulator.

The good news is that you don't have to guess whether you have adequate fuel flow. You can, and should, test it, as outlined in AC 90-89A. You can do a preliminary version of this as soon as you have the wings on, gear on, and plumbing done- you don't even need an engine. It is still crucial to test again right before your first flight, just to be sure that none of your system has become blocked by bug activity or fabrication debris. When I tested mine I got around 25gph with all 3/8 line plus a gold cube sensor. Without the sensor, the flow would have certainly been greater. The guidance in the AC addresses how much flow is enough, with or without a pump. If I were to upgrade to an engine with higher fuel demand, I'd have to remove the sensor or add a pump to be in compliance. If you test yours before and after with 3/8" and 1/2" line and write a Beartracks article about the process and results, I'll gladly buy you some fuel line.

If you were going to use 1/2, it seems to me that the only place to worry about it would be from the valve to the engine. All of the 3/8" lines uphill from there are duplicated at least once, unless you are only running on one tank. I don't think it's a case where bigger is necessarily better. For example, how does the minimum bend radius compare? There is not limitless space, and for me some of the fuel line routes were really tight.

[alaskabearhawk]

A DeHavilland Beaver, with an R985 (450HP MTO, supercharged) uses 1/2 inch fuel lines. I can't even imagine putting 1/2in fuel lines in a Bearhawk. If you think your BH will draw that kind of fuel flow that a P&W radial has on takeoff, more power to ya...no pun intended. I'm doing 3/8ths.

[Sir Newton]

lol. I am not commenting on this thread.

[Mark Goldberg]

For this discussion, I used 3/8" fuel lines for my LSA instead of the 5/16" Bob recommended. I had so many fittings for the 3/8" lines left over from previous projects that I had to buy very little except the fuel line itself. The weight penalty for just the tubing was very minor and possibly the 3/8" fittings weighed a tiny bit more than the 5/16" fittings. Maybe I had a 4 oz weight penalty over all.

When I did my fuel flow test it was 28 GPH when the fuel selector valve was on BOTH. And 25 GPH when just one side selected. Plenty fuel flow for a 540. Mark

[dedgemon]

The 3/8 lines are standard in RV's using 180 - 200 hp engines. These need 16 gallons per hour at takeoff and the lines will flow MUCH more than that with a 5 psi pump. I'm pretty sure they're the standard for Cessna also in their high wing airplanes.

Marks flow test pretty well tells the tail.

[whee]

The Beaver uses a fuel pump and fuselage tanks so I don't think it is a fair comparison.

Imagine being low on fuel and having to do a go around so the remaining fuel is at the back of the tank and forward fuel line is no longer supply fuel. My BH will not have "Both" so I need to be able to supply 25gph though a single fuel line.

[alaskabearhawk]

The Beaver uses an engine driven fuel pump, as anyone should with a 540. Also, there is an advantage to having fuel up in the wings as opposed to belly tanks.

[Mark Goldberg]

Actually Paul - I lot of Bearhawkers run their 540 powered Bearhawks with gravity flow systems. There has never been a problem I have heard of. Mark

[Blackrock]

I asked Bob about this on the phone once and he told me that the 3/8" lines would provide perfectly adequate flow for this engine, though it might not be the 150% the FAA prescribes. He said there was nothing wrong with using 1/2" lines except that they are harder to bend without kinking.

I'm going to give the 1/2" lines a try. Somewhere in dream land I would love to have good enough flow to put in a gravity fed fuel transducer without requiring a fuel pump. I'm also using the ACS fuel selector with 3/8" NPT ports for 1/2" lines. Remember the old axiom, "The only time you have too much gas is when you are on fire."

I have a fuel transducer in a gravity system. The EI Gold Cube works great with very little head loss. It is the only one I know of that works in a gravity fed system.

[Blackrock]

I'm running a O-540 too, BTW